Mass Spectrometry (MS) is a powerful analytical-tool that can provide the information about molecular weights and chemical structures for analytes. Charged gaseous ions are generally generated in an ionization source and subsequently distinguished based on their mass-to-charge ratios in a mass analyzer operated by an electric or magnetic filed.
Both matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) mass spectrometry, which have high sensitivity and wide mass range, are generally used for the analysis of high-polarity and high molecular-weight of analytes. The detectable mass range is up to several hundred thousand Daltons with the detection limit in the low fmol (10−15 mole) to amole range. Therefore, both mass spectrometries have been widely used in the research for life science and proteomics.
MALDI mass spectrometry, which is different from direct LDI MS, requires small organic molecules used as a matrix having the capacity to absorb the laser energy to assist laser desorption/ionization of samples. Therefore, the detectable mass range is extended to a higher mass than that in direct LDI MS. The MALDI results are mainly determined by the selection of matrices. However, high matrix background appearing in the MALDI mass spectra, the requirement of co-crystallization of analytes with matrices, and analyte signals only found in “sweet spots” all arise as conventional MALDI matrices are used for MALDI MS analysis.
Using inorganic materials as the assisting substrate in MALDI MS analysis can avoid some problems arising as conventional matrices are used. Tanaka et al. are the pioneers, who used inorganic material mixing with glycerol as the matrix. They employed cobalt powder (˜30 nm) mixing with glycerol as the assisting matrix in MALDI MS analysis for protein analyses. Later on, Sunneret al. alternatively used micro-sized graphite powder mixing with glycerol as the matrix. They also termed this approach as surface-assisted laser desorption/lonization mass spectrometry (SALDI MS). Graphite powder is proposed as the energy transfer medium during SALDI MS processes.
A remarkable progress in the development of inorganic material-assisted laser desorption/ionization, which is called desorption/ionization on silicon (DIOS), was made by Siuzdaket al. in 1999. DIOS is a matrix-free method, which uses a porous silicon film capable of absorbing the laser energy as the sample deposition film. The porous silicon substrate is facilitated by treating the silicon surfaces electrochemically and mass analysis using silicon films that are formed from a silicon surface by plasma-enhanced chemical vapor deposition, have been applied successfully to the analysis of small molecules However, the surfaces of the porous silicon substrates are easily oxidized, which may lead the substrates become ineffective. The unstable feature of the porous silicon film may cause problem in storage and in practical uses. In addition, the fabricaton of the porous silicon substrates required particular equipments for production of porous surfaces though some commercialized products are already available. Generally, the upper detectable mass range is ca. 6 kDa. A laser desorption/ionization mass spectrometry using a stable assisting material with the advantages of low matrix background, ease of sample preparation, homogeneous sample deposition, stable substrate surface, and wide detectable mass range should be desirable.